This invention relates to digital speech vocoders and more particularly to a pitch period extraction algorithm and an implementation to carry out the same for such vocoders.
One of the most difficult problems in vocoders is the reliable determination of the pitch period of voiced speech. A great deal of work has been done in this area in the past, resulting in many pitch extraction techniques. However, the basic operating principles of these many pitch period extraction schemes fall into one of the following three categories:
1. Direct analysis of a speech spectrum or a processed version of the spectrum, e.g. cepstrum. PA1 2. Direct analysis of the time domain speech wave form or a processed version of the time speech wave form, e.g. filtering and cubing the speech. PA1 3. Analysis of an averaging function obtained from the speech spectrum or time speech wave form, e.g. the auto-correlation function of the speech.
When approaching the task of devising and implementing a pitch extraction algorithm a major objective is to develop a system of good performance with a minimum of hardware complexity.
The method of achieving this objective is greatly influenced by the ultimate purpose of the device. In general, a pitch period extractor is used as part of a large system for speech analysis. When this is true, the most effective method of attaining this objective from a systems point of view is to try to utilize existing data from other parts of the system as an aid in accomplishing the task of pitch period extraction.
The pitch period algorithm and implementation of the same as described herein is part of a speech analysis system. The purpose of the system is to represent speech signals in terms of a small enough number of parameters so that digitized speech can be transmitted over a digital communication channel at transmission rates as low as 2400 bits per second with the ability to regenerate speaker recognizable speech at the speech synthesis or receiver portion of the system. Due to the processing performed in this system the available data makes the time domain approach to pitch period extraction far simpler than the other two methods mentioned hereinabove.